484, E. A. MINOCHIN. 
logical details, I will state merely that these nuclei exactly 
resemble in size, structure, and appearance the nuclei of the 
remaining ectoderm, and differ in precisely these three points 
from the nuclei of the endoderm, still more so from the nuclei 
of the amceboid mesoderm-cells. The granules of the cells 
appear when carefully focussed as round black spots, but 
when the microscope is a little high or too low they appear as 
minute black rings round a central clear spot, and in sections 
often look not unlike fibrillz cut transversely, which of course 
is not the case. 
Thus, to recapitulate, this sphincter is composed of two 
layers of ectoderm, with a few scattered amceboid cells be- 
tween, and the contractile cells are the ectodermal epithe- 
lium. Thus, in one of the simplest existing types of sponges, 
I have arrived at the same result as Topsent,! who in his 
work on the Clionide, finds that the ectodermal or endo- 
dermal “‘ cellules de revétement”’ are the contractile elements. 
The sphincter of the oscula of Leucosolenia clathrus is 
an especially favorable object in which to study this question, 
as the cells are so large compared with the minute cells of 
siliceous sponges, and the sphincter itself can be so easily 
prepared out or cut into sections. We have in this sphincter 
perhaps the most primitive type of muscle-cell in the animal 
kingdom ; it can hardly be called a myo-epithelial cell, it is still 
a simple ordinary epithelial cell. 
Various authors” have described muscle-cells lying in the 
mesoderm; and until Topsent wrote, I think I am right in 
saying that muscular cells in sponges were regarded as meso- 
dermal. There is no reason why, in a highly differentiated 
sponge, muscle-cells originally forming part of an epithelium 
should not become more specialised and sink into the meso- 
1 “Contributions a l’étude des Clionides,” ‘ Arch. de Zool. expér. et gén.,’ 
tome v bis, Suppl. (1877—1890), p. 24, et seq. 
2 Vide Sollas’s article “ Sponges,” ‘ British Encyclopedia ;? ‘‘ Monograph 
of the Tetractinellida,” ‘ ‘ Challenger” Reports,’ p. 42; von Lendenfeld, 
‘Beitrag zur Kenntniss des Nerven- und Muskel-systems der Horn- 
schwamme,” ‘S. B. k. pr. Akad. Wiss.,’ Berlin, Nov. 12th, 1885. 
